1. Field of the Invention
The present invention relates to the fields of waveguides and coupling mechanism for extracting/injecting a signal of single/two orientation from a guide conveying information in two orthogonal orientations. The present invention specifically relates to a waveguide Ortho Mode Transducer (OMT) for extracting waves of orthogonal polarizations from a main guide.
2. Description of the Prior Art
A number of schemes are in vogue for extracting/injecting a signal of a particular orientation from a mixture of two orthogonal orientations. Generally, such schemes have coupling apertures aligned to the respective field to be extracted. The coupling apertures are placed along the periphery of the waveguide for extraction of a wave of the particular orientation, with a similar configuration for the orthogonal signal. In general, the slots are disposed at an angular interval of 90 degree around the main guide corresponding to the signals that are spatially aligned in quadrature and hence at an angular interval of 180 degree for similar polarization.
In addition, the main guide is tapered so as to result a cut-off region for the signals of both the orientations, which ensures the reflection of any signals that fails to couple directly through the apertures. The placement of the apertures at a predetermined location ensures the coupling of the reflected signal. The conventional art for coupling of the orthogonally aligned waves differs in placement of the coupling apertures that being on the periphery of the main guide.
Referring to FIG. 1a and 1b, 3-dimensional and side views of an ortho mode transducer with coupling apertures in the axis of a main guide are illustrated, respectively, in accordance with prior art. The main guide of circular cross section comprises two ports 1 and 3 at both ends, of a predetermined dimension so adjusted that the requisite band of frequencies are communicated without hindrance to the intended networks. The ports 1 and 3 are placed at a predetermined distance from each other, which ensues in a taper section 2 between the ports 1 and 3. Four coupling apertures 4 are placed parallel to the axis of the information conveying main guide. The main guide of circular cross section encloses the information conveyed in spatially orthogonal waves designated herein as H and V for the horizontal and vertical orientations, respectively.
The main guide of circular cross section is bridged to the related RF networks through the port 1, at which both the V and H signals are available for processing by the polarization discriminator. The port 3 of the main guide is so configured that the guide remains at cut off in the frequencies of interest, and hence projects a virtual short. It is evident to those skilled in the art that the proper placement of this gradual virtual-short causes the reflected waves to be in phase with the incident waves, thus ensuring maximum coupling.
Moreover, the coupling apertures 4 communicate to the external networks via branching waveguide 5 also disposed in a symmetrical and/or orthogonal manner around the main guide. In order to combine the waves from each of the branching waveguides 5, the coupling apertures 4 corresponding to same polarization are connected to a hybrid network composed of waveguide circuit elements such as Magic-T (not shown).
Referring to FIGS. 2a and 2b, 3-dimensional and side views of an Ortho mode transducer with coupling apertures in a taper portion 2 of a main guide are illustrated, respectively, in accordance with prior art. The coupling apertures 4 are aligned in a manner parallel to the tapered portion 2 of the main guide. In order to increase the bandwidth, the coupling apertures 4 are placed along the tapered portion 2 of the information conveying main guide. In addition, the two ports 1 and 3 at both ends of the main guide are of different dimension and are spaced apart by a predetermined distance. The orthogonal polarizations are extracted from the main guide via the coupling apertures 4 and communicated to the external network by the branching waveguides 5. The combination of the waves of like polarization from the respective branching waveguides 5 is effected by the use of waveguide circuit elements such as Magic T.
An innate problem with the prior art is that the coupling aperture being of an electrically resonant nature limits the bandwidth that can be attained. An attempt to solve the same was effected by differing in the placement of the slot along the information conveying main guide.
It is evident to those skilled in the art that such tapers and coupling apertures are potential sources of unwanted higher order modes, which deteriorate the performance at higher frequencies and the same circumvented by the use of symmetrical branching structures.
In summary, the existing symmetrical branching OMTs are limited to narrow band applications and therefore what is needed is a symmetrical branching OMT that can be made to perform satisfactorily for moderate to wide band applications.